Epoxy Resins

2 Fluorine-Containing Epoxy Components and Plastics JAMES R. GRIFFITH and JAMES E. QUICK

Downloaded by EAST CAROLINA UNIV on March 17, 2016 | http://pubs.acs.org Publication Date: June 17, 1970 | doi: 10.1021/ba-1970-0092.ch002

Naval Research Laboratory, Washington, D. C. Tetrafluoro-m-phenylenediamine reacts with epoxy resins more slowly than m-phenylenediamine to produce plastics with comparable properties. In combination with 1,4-cyclohexane diepoxide, it produces an exceptionally rigid plastic with improved water resistance. Glycidyl ethers can be prepared from 4,4'-dihydroxyoctafluorobiphenyl and tetrafluororesorcinol which react to produce plastics with the fluorinated diamine. Partial esters of trimesic acid or trimellitic anhydride and aliphatic fluoroalcohols cure epoxy resins to produce hydrophobic plastics. "Oecent advances in the chemistry of heavily fluorinated compounds have made possible the syntheses of some epoxy plastics which contain appreciable amounts of fluorine. There is little information i n the literature concerning multifunctional components which are suitable as epoxy intermediates of high fluorine content. The effects of the halogen upon the reactivities of components and the properties of plastics is essentially unknown for the case of densely crosslinked systems of the usual epoxy type. It may be possible to produce materials with the convenient use properties of epoxies which "cure" to yield plastics with such fluorocarbon properties as non-wetting by water, low suface friction, and high thermal stability. This report is concerned primarily with some fluorinated components suitable for making epoxy plastics and with some effects of water upon these plastics. Epoxy reactions of the coupling type in which an active hydrogen atom adds across the epoxy ring generally produce stronger plastics than catalytic reactions which produce polyethers. A widely used reaction is the addition of an amino hydrogen across the epoxy ring: A

V

— N — H + C H , — C H — -> — N — C H — C H — 2

OH 8 Lee; Epoxy Resins Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


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Fluorine-Containing Plastics

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Since the co-reactant, or ' curing agent," becomes as integral a part of the plastic molecular network as the epoxy component, it is necessary for both to be fluorinated if the plastic is to have the highest possible content of fluorine. Plastics of high strength are commonly produced from aromatic diamines, and three such materials are now available commercially upon which the hydrogen atoms of the aromatic ring have been replaced by fluorine. These are tetrafluoro-m-phenylenediamine ( F M P D A ) , the para isomer, and 4,4'-diaminooctafluorobiphenyl. Of the three, F M P D A is the best curing agent for epoxies because it is the lowest melting, most soluble in resins, and most reactive. The fluorinated aromatic diamines react more slowly with typical epoxy resins than the unfluorinated analogs. This probably results from the electron-attracting property of fluorine which is transmitted to the amino groups and makes them less nucleophilic. The normal m-phenylenediamine ( M P D A ) reacts to stage (a semi-cured, brittle state) with epoxies of the diglycidyl ether type at 25 °C. within 24 hours, but F M P D A reacts only with the most aggressive resins at this temperature. A cure temperature of 160 °C. for 16 hours or more is usually required for the fluorinated diamine to effect a thorough cure. The plastics so produced from the diglycidyl ether of bisphenol A with the fluorinated and unfluorinated M P D A have heat distortion temperatures ( A S T M D648-56) of 146°C. and 1 5 0 ° C , respectively, which indicates that they are of comparable rigidity and crosslink density. The F M P D A has been studied as a curing agent for 1,4-cyclohexane diepoxide. This small-molecule epoxy resin is cured by aromatic diamines to produce some of the most rigid plastics known with mechanical moduli of about 1,000,000 p.s.Lg. It is an unusually reactive resin and is converted to stage by F M P D A at room temperature i n about six weeks. A major disadvantage of the plastics produced from 1,4-cyclohexane diepoxide is that they contain high concentrations of hydrophilic hydroxyl and amino groups which render them sensitive to damage by water. Some improvement i n this defect can be realized by substituting F M P D A for the more water-soluble M P D A as the curing agent, as illustrated i n Figure 1. The fluorine-containing plastic does not spall as readily upon being removed from boiling water as the normal material, but it is also exceptionally water-absorptive (Figure 4). Some of the mechanical properties of the plastic produced from 1,4-cyclohexane diepoxide and F M P D A i n stoichiometric amounts are as follows: Heat distortion temperature ( A S T M D648-56), 208°C. Compressive yield ( A S T M D695-63T) 42,000 p.s.i.g. at 15% deflection. Compressive ultimate, 72,000 p.s.i.g. at 46% deflection. Tensile ultimate ( A S T M D638-64T) 14,000 p.s.Lg. at 2 % elongation. Tensile modulus, 900,000 p.s.i.g. Compared with a common M P D A - c u r e d diglycidyl ether

Lee; Epoxy Resins Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


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Figure 1. The effect of short-term exposure to boiling water upon 1,4-cyclohexane diepoxide cured with fluorinated and unfluorinated m-phenylenediamine Right: FMPDA Left: MPDA

of bisphenol A , the heat distortion temperature of this plastic is some 60 °C. higher, its compressive strength properties are double, and its elongation is about one-third. Fluorine on the aromatic rings of the curing agent appears to have small effect upon the thermal stability of plastics produced from bisphenol-A diglycidyl ether or 1,4-cyclohexane diepoxide. Table I shows that major decompositions occur i n air between 240° and 280°C. Table I. Weight Losses in Air of 1.5-Gram Solid Plastics Cured with FMPDA during 5 Hours' Exposure at Each Temperature Bisphenol A Diglycidyl Ether

Cyclohexane Diepoxide

Temperature, °C.

Progressive

Single Exposure

Progressive

Single Exposure

200 220 240 260 280 300

0.26% 0.47 0.67 6.2 22 32

0.26% 0.31 0.55 2.1 21 32

0.30% 0.45 0.49 25 33 34

0.30% 0.34 0.51 28 29 59

Because of the availability of the fluorinated aromatic diamines, the syntheses of some similar diglycidyl ethers with fluorine i n place of hydrogen on the aromatic rings was undertaken. T w o suitable phenolic


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intermediates are 4,4 -dihydroxyoctafluorobiphenyl and tetrafluororesorcinol ( I ) . The biphenyl derivative reacts with excess epichlorohydrin i n the presence of alkali to produce a diglycidyl ether in good yield (2, 4). /


F

F

F. F

This material was purified by recrystallization from methanol. It is a white, crystalline solid with a melting point of 7 5 ° - 7 7 ° C . The theoretical epoxy equivalent weight is 221, and a value of 242 was determined by the pyridinium hydrochloride in pyridine method. Anal Calcd. for C H F O : C, 48.88; H , 2.28; F, 34.36 Found: C, 49.15; H , 2.50; F, 34.43 1 8

1 0

8

4

The infrared spectrum of this material (Figure 2) was determined on the solid in a compressed potassium bromide window. Prominant features of the spectrum include the C — F bands between 9.0 and 10.5 microns and the epoxy band at 11.1 microns.

Figure 2.

Infrared spectrum of 4,4 -diglycidyl ether of octafluorobiphenyl

Tetrafluororesorcinol reacted readily with epichlorohydrin before any alkali was added. Most of the product in this case was a chocolatebrown resinous solid. This was extracted with benzene after most of the epichlorohydrin was removed by vacuum evaporation. Benzene was then removed by vacuum evaporation, and the liquid residue was vacuum distilled at 170°-175°C. (1 mm.) in a Hickman still. A clear, liquid product was obtained in low yield which contained glycidyl groups and 22.69% fluorine. A pure product was not isolated, but the epoxy equivalent weight of 250 indicates that the material may be principally the dimer (Theoretical E . E . W . = 2 5 6 ) .


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F

F

F CH —CHCH —O 2

2

F —O—CH CHCH —O 2

F

F F

2

OH

F

O—CH^CH—CH

V

2

v o


RESINS

o

F M P D A cures these fluorinated resins at 165 °C. during 24 hours to produce plastics. The maximum fluorine-to-carbon ratio by number of atoms on a disubstituted aromatic ring is 2-to-3. Although these plastics carry fluorine on all available ring positions, they are not markedly hydrophobic nor of low surface friction. In Figure 4, it can be seen that the water absorption of the plastic produced from the diglycidyl ether of octafluorobiphenyl and F M P D A is only slightly less than that of a common epoxy plastic. The presence of perfluorinated aliphatic units within the molecular network of the plastic is apparently necessary if properties similar to those of fluorocarbon polymers are to be realized in epoxy systems. The polar hydroxyl and amino groups within the network detract from hydrophobic properties, and the glycidyl region appears to be unaffected by aromatic fluorocarbon within the structure. The strength properties of epoxy plastics depend upon the development of a tightly crosslinked, three dimensional molecular network during the cure reactions. The inclusion of perfluorinated aliphatic units into such a network should be accomplished ideally i n such a manner that the compact, functional nature of the components is not impaired. Numerous — C F groups on aromatic resins and curing agents would constitute a favorable means of including aliphatic fluorocarbon. However, the chemistry of fluorine compounds has not advanced to the point that many d i - or tetrafunctional intermediates heavily laden with trifluoromethyl groups are known. 3

Figure 3. Water retracts from a bonded joint adhered with an epoxy containing perfluoroaliphatic chains


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In order to obtain an indication of the effects of perfluorinated aliphatic groups upon the behavior toward water of epoxy plastics, some polyfunctional fluoroester curing agents for epoxy resins were synthesized. Esters of 1 H , lH-fluoroalcohols have been reported to be exceptionally stable to hydrolysis and pyrolysis (3), and the synthesis from acid chlorides and alcohols occurs readily. For example, the esteranhydride of trimellitic acid was prepared as follows:

The acid chloride function is sufficiently more reactive with these fluoroalcohols than the anhydride function that the selective esterification can be accomplished, leaving the anhydride for subsequent use i n curing epoxy resins. A similar reaction with trimesoyl chloride, followed by hydrolysis of the excess acid chloride, was used to prepare partial esters with acid functions for use i n curing epoxies. CI

c=o C1C ^ ^ C C l

o HOOC

C—O—CH (CF ) CF 2

2

B

3

HOOC Partial esters of lH,lH-heptafluorobutanol and lH,lH-pentadecafluorooctanol were prepared from the trimellitic anhydride and trimesic acid i n


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this manner. The products were difficult to obtain i n high purity because of unusual solubility characteristics and the similar nature of the contaminants, such as the diester of trimesic acid. However, partial esters of sufficient purity to function as epoxy curing agents were obtained. The ester anhydrides of trimellitic acid are lower melting and more soluble i n epoxy resins than are the partial esters of trimesic acid. During the cure reactions, the long pendant fluorocarbon chains apparently concentrate i n the resin surface and render the cured plastic hydrophobic. Figure 3 shows a lap bond between two steel panels adhered with a composition of the diglycidyl ether of bisphenol A and the l H - l H - p e n t a decafluorooctyl ester of trimellitic anhydride. Water applied to the bonded region retracts from the adhesive on to the steel, or stands i n beads with high contact angles as illustrated.

WEEKS

OF

IMMERSION

Figure 4. Water absorption at 25°C. of the following plastics: (A) 1,4-cyclohexane diepoxide and tetrafluoro-m-phenylenediamine; (B) diglycidyl ether of 4,4'-dihyaroxyoctafiuorobiphenyl and tetrafluoro-m-phenylenediamine; (C) diglycidyl ether of bisphenol A and mono-pentadecafluorooctyl trimesic acid; (D) diglycidyl ether of bisphenol A and pentadecafluorooctyl trimellitic anhydride; ana(S) diglycidyl ether of bisphenol A and aromatic diamine Fluoroaliphatic chains within an epoxy plastic apparently render it resistant to the internal penetration of water also. Curves C and D i n Figure 4 show the relatively low water absorptions of bisphenol A resin cured with the trimesic and trimellitic fluoroesters respectively. The amount of weight increase of these plastics during six weeks' immersion i n distilled water at room temperature is about half that of the standard


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arnine-cured material. These data were detennined on discs of plastic approximately the diameter and thickness of a nickel coin. The inclusion of large amounts offluorocarboninto epoxy resins in an optimum manner is dependent upon further developments in the chemistry of heavilyfluorinated,polyfunctional intermediates. If the ratio of polar groups tofluoroaliphaticunits can be reduced in the cured plastics, and thefluorocarboncan be located close to and distributed along the molecular network units, then epoxy plastics with outstanding resistance to the effects of water should be obtained. Acknowledgment The authors wish to acknowledge the assistance of Arthur G. Sands in preparing and testing plastic samples. Literature Cited (1) Air Force Materials Laboratory, Tech. Rept. AFML-TR-65-13, Part III (April 1967). (2) Erich, W., Bodnar, M. J., J. Appl. Polymer Sci. 3, No. 9, 296 (1960). (3) Faurote, P. D., O'Rear, J. G., Ind. and Eng. Chem. 49, No. 2, 189 (1957). (4) Kelly, P. B., Landua, A. J., Marshall, C. D., J. Appl. Polymer Sci. 6, No. 22, 425 (1962). RECEIVED

June 10, 1968.


Epoxy Resins

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